The present invention relates to apparatus and method for testing blood and more particularly to the determination of the percentage of carboxyhemoglobin in blood.
There continues to be a pressing need for reliable methods for measuring the percentage of carboxyhemoglobin (COHb) in blood outside the laboratory under adverse conditions in the field. Traditional methods such as gas chromatography are too demanding for routine use in such situations and require large, precisely calibrated equipment, which as well as requiring substantial time to perform the testing, is unable to withstand usage in the field, and requires laboratory conditions to maintain calibration.
Additionally, testing performed on unconscious persons or newborn children require that only a small amount of blood be used. Spectrophotometric methods based on absorbance measurements lack the sensitivity necessary for small sample volume and often require extensive testing time.
It is well established in the medical literature that COHb levels as low as 10 percent have been proved to have deleterious effects on human performance. In high carbon monoxide situations such as in fire-fighting procedures where the fire-fighters and victims of the fire are exposed to high carbon monoxide levels, it is important that carbon monoxide levels in the blood be readily established. Such is particularly important aboard a ship where the fire gases can be spread through the ship by the ventilating system and especially where a ship deployed at sea is isolated from extensive medical care facilities.
Prior art spectrophotometric apparatus and methods have proven to be unsatisfactory for this need. It is particularly important that the testing be accomplished with a portable instrument on a very small amount of blood, such as derived from a prick on a finger and that the time for testing be short, preferably two minutes. The method of Commins and Lawther in their published article "A Sensitive Method for Determination of Carboxyhemoglobin in a Finger Prick Sample of Blood" 22 British Journal of Industrial Medicine 139 (1965) requires a supply of oxygen and at least 20 minutes to bubble the oxygen through a blood sample. Additionally, the testing must be conducted at four wavelengths which do not provide the required sensitivity and great care must be taken to prevent air contamination of the samples. The method disclosed in the article of Klendshoj, Feldstein, and Sprague entitled "The Spectrophotometric Determination of Carbon Monoxide" appearing at 183 Journal of Biological Chemistry page 297 (1950) uses light wavelengths of 555 and 480 nanometers with one of the wavelengths being an isobestic point formed by the intersection of the absorbance-wavelength curves of COHb and Hb, provides only one one-hundredth of the desired sensitivity, and requires a relatively larger amount of blood. The article by Small, Radford, Frazier, Rodkey and Collision entitled "A Rapid Method for Simultaneous Measurement of Carboxy and Methemoglobin in Blood" appearing at 31 Journal of Applied Physiology at page 154 (1971) tests the sample at four wavelengths, demands great care to prevent contact with air, is dependent upon the pH of the solution, and is critical to the light wavelengths used. Brown et al U.S. Pat. No. 4,134,678 requires relatively larger amounts of blood for testing and does not lend itself to portable use. Thus, the above methods and apparatus do not lend themselves for use outside of the laboratory and performance by other than highly specially trained personnel or for testing unconscious persons, newborn children, or small animals.
Accordingly it is desirable to provide apparatus for testing the percentage of COHb in blood quickly, using a smaller quantity of blood, at greater sensitivity which can be quickly accomplished with a portable instrument outside the laboratory under adverse conditions by para-medical personnel. It is further desirable to provide spectrophotometric apparatus for testing COHb in blood at wavelengths wherein the absorbance of the components are maximized.
It is important that the calibration of such a portable instrument subject to rough handling be minimized. Accordingly it is desirable to minimize calibration requirements and to ascertain the desired values by comparison to predetermined data obtained from high accuracy laboratory equipment.